(1) Field of the Invention
The present invention relates to the field of textile structures and more specifically those which are adapted to the manufacture of composite materials. The invention relates more specifically to a textile structure which is capable of deformation and can thus be shaped in an advantageous manner at the time of a molding process for obtaining a final composite material.
(2) State of the Prior Art
It is known that composite materials are constituted by a textile reinforcement and by a resin matrix, this unit being made and shaped at the time of a molding process. It is not necessary to go over in detail the various techniques which can be used in practice for the manufacture of such composite materials because they are now well-known to one skilled in the art.
The manufacturers of composite materials are well aware that the shape of the final pieces to be obtained often raises technical problems which are difficult to solve. In fact, the textile reinforcement consequently has to be shaped either in advance or at the time of positioning in the mold. When it is a matter of manufacturing pieces of developable shape, the textile reinforcement elements can be prepared without major difficulty, except for allowing a good injection of resin into the mold before the polymerization which leads to the final composite material. The technical difficulties are therefore dependent upon the nature of the composite material itself and not upon its shape. In complete contrast, when it is necessary to manufacture non-developable pieces, use has to be made of separate elements of textile reinforcement, which it is therefore appropriate cut, position and to superpose so as to provide a reinforcement corresponding to the final shape desired. Any cut-out in a textile reinforcement introduces a discontinuity in the yarns and fibers thereof, which leads to regions of weakness in the piece made of the composite material. This is completely unacceptable for certain applications, for example in aeronautics. Moreover, the cutting-out operations are lengthy and costly
The Patent FR-1 068 334 relates to a process for obtaining a fibrous material intended for the reinforcement of plastic materials. The aim of this patent is to make a network of a number of fabrics, imbricated with one another by a weaving operation.
The imbrication between the fabrics is obtained by warp yarns which pass periodically from one fabric to another.
The fibrous material obtained by this process makes it possible to limit the risks of cleavage which can exist when two adjacent layers of fabric are not connected to one another by a fibrous link.
However, precisely because of the imbrication between the fabrics, the fibrous material obtained is not deformable and therefore cannot be shaped in an advantageous manner.
Efforts are therefore currently being made to perfect textile structures which can be deformed so as to significantly reduce the implementation time, making it possible to manufacture a piece made of a composite material while avoiding cut-outs, the textile structure then being capable of being preformed in order to be adapted to the shape of the piece to be obtained.
It is this technical problem which provides the basis of the present invention. The invention affords a simple solution and produces very advantageous results in relation to the proposals of the prior art.
It has already been proposed to use, as textile reinforcements, fabrics of a somewhat loose texture, in such a manner that they give at the time of shaping. However, to the knowledge of the applicant, this solution has never led to satisfactory practical results because, if a structure is deformable, it does not, for all that, satisfy the other criteria which are necessary in order that it constitute a good textile reinforcement for composite materials.
By way of a simple example, the mats which are constituted by yarns and fibers distributed in a random manner are easily deformable but have very poor mechanical properties. Similarly, fabrics of the satin type are not suitable either. Despite a certain capacity for deformation, such a fabric forms creases if it is attempted to position it on a complex surface which has particularities. Thus, the presence of creases considerably weakens the mechanical properties of the piece made of the composite material.
It has already been proposed to use fabrics comprising yarns in one direction and, in the other direction, undulating yarns, this undulation being oriented in the plane of the fabric. Such a textile structure has the capacity of being deformed, at least in a direction parallel to the axis of the undulations. However, after deformation, the orientation of the fibers is fundamentally modified. Therefore does not allow the properties of composite materials which have been made and contain such structures as reinforcements to be anticipated. It is also known that fabrics exist which comprise undulating yarns, the undulations of which are perpendicular to the plane of the fabric. These fabrics are deformable, at least in the direction of the undulations, but they too do not provide an optimum orientation of the fibers when they are shaped in order to serve as a reinforcement in composite materials. As in the case of the undulations in the plane, but to a lesser degree, such fabrics are not suitable for high-performance composite materials.
It has further been proposed to use, as textile structures, fabrics obtained by oblique interlacings of yarns, so as to make somewhat loose structures, similar to lattices, of which the intervals separating the yarns have a shape similar to lozenges or parallelograms, having a certain capacity for deformation. However, these structures are very loose, and it is not possible to obtain fiber coverage rates in the textile reinforcement which are adequate in order to obtain good properties in the composite material. By fiber coverage rate is understood, in general, the ratio of the volume occupied by the fibers in relation to the volume of the textile surface.
Textile structures are also known which are obtained by superposing a certain number of fabrics or layers in which the yarns and fibers have a certain capacity for sliding in relation to one another. Thus there exist structures in which the fibers can, at the time of deformation, orient themselves in a direction which is generally parallel to that of the plane of the structure as a whole. This structure of a multi-directional type is also not suitable as a reinforcement for composite materials, because when it is used to make pieces which have sharp angles, the sliding of the fabrics leads to a perforation of the fabric which is, of course, unacceptable.
Lastly, it has already been proposed to use superposed fabrics or layers, generally two in number, or at most three, which are kept in position by very loose tying so as to allow a sliding of the yarns and fibers parallel to the plane of the structure as a whole, so that, at the time of shaping on a curved surface, the yarns essentially follow the contour of the shape. Although the general concept of this solution appears good, it has not given rise, thus far, to a satisfactory practical embodiment, above all because of poor performances obtained upon use as textile reinforcement. Thus, the deformability can be obtained in a satisfactory manner, but the performances of the structure in the dry state, and above all after shaping and impregnation, are not adequate for the requirements of reinforcements for high-performance composite materials.
In order to clarify these ideas, if an arbitrary scale is adopted in order to evaluate performance after impregnation of the structure, and by attributing the value 10 to a mat, as far as mechanical performances are concerned, a value close to 12 can be estimated for the results provided by a structure with fabric comprising undulations in the plane, a value of around 14-15 for fabrics with undulation perpendicular to their plane and likewise for lattice-type fabrics. As far as fabrics are concerned which allow a parallel sliding of the fibers, it is not currently possible to reconcile at the same time a good aptitude for deformation and adequate mechanical performances. Although, by adopting the above-mentioned arbitrary scale, such fabrics make it possible to achieve values of 18-20, they are then no longer sufficiently deformable. Conversely, when, with a sufficiently loose tying, an adequate deformability is obtained, the mechanical performances do not exceed a value of 14-15, as in the case of a lattice-type fabric.